An ultrasonic imaging apparatus is designed to transmit ultrasonic waves to an object to be examined through an ultrasound probe that is applied to a body surface of the object, and to receive reflected waves (echo signals) from inside of the object. In this way, it is possible to display cross-sectional images of portions of the object on the basis of the echo signals for use in medical diagnosis and the like.
Generally, an ultrasound probe is formed by arranging a plurality of transducers at even intervals in a straight line, a curved line, or a plane. For example, in an ultrasonic imaging apparatus of the linear scan type, an aperture is formed by selecting a transducer group in which the transducers are to be driven at the same time by an aperture selecting switch, and this aperture is sequentially moved so as to scan the ultrasonic beams inside an organism or an inorganic object. Scanning is performed in the same manner as that used in an ultrasonic imaging apparatus of the convex scan type.
In both the apparatuses of the linear scan type and of the convex scan type, when a direction or an angle of the ultrasonic beam is predetermined, a plurality of received beam signals acquired by the scan are stored in memory cells at addresses set in correspondence with each beam direction or angle, and an image is displayed.
On the other hand, in an ultrasonic imaging apparatus of the compound scan type, as disclosed in Japanese Unexamined Patent Publication No. JP-A-64-62133 and JP-A-5-285146, a probe is moved along the surface of the object, and an image is obtained from a received beam signal corresponding to an ultrasonic beam signal at various angles and at various positions, and these images are combined in real time. In this case, because the positions and the angles of the ultrasonic beams vary, the position and the angle of the received beam signal are calculated by detecting positional information of the probe in real time, and then images are synthesized on the basis of this information. Accordingly, by superposing the same cross-sectional images, a boundary between mediums in parallel with an ultrasonic beam and inside of an object to be examined, which boundary is only insufficiently depicted through a cross-sectional image in a single direction, can be clearly depicted. Further, a dark portion appears due to a small luminance signal caused by multiple reflection or behind a strong reflection body. There is an effect that the image of such a portion is improved by superposing images taken from various directions.
However, according to the above-described conventional method of compound image generation, image synthesis is performed after converting the received beam signals into luminance signals. Therefore, the method does not demonstrate an effect of improving the signal to noise ratio (S/N ratio) in the known opening synthesis by superposing received beam signals having phase information and obtaining the resulting interference.
Besides, since an ultrasonic image can be obtained only within an effective FOV (field of view) of the probe, for example, the image of a portion of an object to be examined having a size larger than the effective FOV can be obtained through a plurality of ultrasonic images which are obtained by moving the probe in a longitudinal direction and joining these images. The obtained image is referred to as a panoramic image, and such a technique is disclosed in Japanese Unexamined Patent Publication No. JP-A-2001-104312.
In the above-described method of panoramic image generation, the angle and position of the probe are detected and the ultrasonic images are joined to make a panorama image. However, since the panoramic image synthesis based on detection of the angle and position of the probe deals with luminance signals, signal processing in consideration of phase signal components of echo signals is not considered in the processing the signals.
An object of the present invention is to improve the S/N ratio of a compound image in an ultrasonic imaging apparatus.